This invention relates to hydraulic master cylinder assemblies used for, but not exclusively for, the braking and clutch systems of automobiles.
In a typical braking system, the hydraulic fluid housed in the reservoir of a master cylinder, is fed into the braking system to replenish fluid lost due to leaks, or to top up the system as it expands due to for example, wear on brake linings in the case of a braking system. The feeding of fluid into the high pressure side of the master cylinder actuating piston, when the piston is at the brake release position is called "breathing" and typically involves a one way valve mechanism, so that fluid can feed freely from the reservoir into the braking system when the brakes are fully released, but fluid is not displaced into the reservoir when the brakes are applied.
It has been propsed in European Patent Publication No. 0008863, to provide a hydraulic master cylinder with a one piece piston, said piston having a breather valve operated by a pivotted lever mounted on the piston and an abutment associated with the cylinder. The pivotted lever is mounted in a wedge shaped slot in the piston and forms a stop for an axial on the breather valve, so that when the piston is in its fully retracted position and the lever abuts the abutment associated with the cylinder, the breather valve will be held open, but when the piston moves upon actuation of the master cylinder, movement of the lever will allow the breather valve to close.
This arrangement provides a method of operating a "breathing" means for a one piece piston by use of a relatively short abutment. An advantage of this construction is that a very deep co-axial blind bore can be made in the piston to provide lateral support for the input rod which will reduce the likelihood of the piston tending to tilt during application of the braking load.
With the construction disclosed in the European Patent Publication No. 0008863, the piston comprises a spool having two axially separated cylindrical portions each of which seals against the wall of the cylinder and an intermediate portion of cruciform section in which are formed an axial groove which co-operates with the abutment associated with the cylinder to guide the piston in the cylinder and a wedge shaped slot to accommodate the lever. Because of its complex shape, the piston is difficult and expensive to manufacture.
The present invention is a modification to the hydraulic master cylinder disclosed in European Patent Publication No. 0008863 in which the construction of the piston is simplified.